Various thermal probe assemblies are known in which a probe portion encloses a thermistor for sensing the temperature of a medium in heat transfer relationship with the probe portion. One such assembly includes a metal outer shell that is crimped over a plastic connector cover. The plastic connector cover supports terminals welded to wire leads of a thermistor located within either a sealed probe portion on the metal outer shell in the case of a liquid temperature sensor, or in a plastic tube supported in the metal outer shell. The plastic tube has an open end for sensing gas temperature. A gasket is located between the connector cover and the crimped edge of the metal outer shell to environmentally seal the thermal probe assembly and a cardboard insulating shell is located between the thermistor and the metal shell to electrically insulate therebetween.
While satisfactory for their intended purpose such metal shell sensor assemblies are expensive to manufacture due to the large number of sub-components which must be individually positioned during the assembly process. Furthermore, the crimped edge that attaches the connector cover to the metal shell is a potential leakage path for contaminants into the interior of the probe if the sealing gasket is not properly compressed on assembly or if it takes a compression set in use.
Another type of probe assembly includes an all plastic housing. Such assemblies eliminate the need for crimping and sealing a metal shell to a plastic connector cover. Such plastic housings are made by a multishot, in place, over molding technique. Such techniques require elaborate molding apparatus to assure that the thermistor element remains properly centered and axially positioned with respect to the probe portion of the assembly as various mold layers are injected to form the housing. Furthermore, such probe assemblies have an overmold layer injected against solder joints formed between thermistor wire leads and the thermistor and/or the wire leads and metal terminals. Such plastic overmolding can impose high temperature and injection pressures on the solder joints to cause the solder to reflow so as to weaken the electrical connection between the thermistor and the terminals.
Furthermore, the interface between layers of over molded material is a potential leakage path for flow of contaminants into the interior of the probe assembly.